Long-term prisoner turning up in secure files (5)
I believe the answer is:
lifer
'long-term prisoner' is the definition.
(I've seen this before)
'turning up in secure files' is the wordplay.
'turning up' says the letters should be written backwards.
'in' indicates the answer is hidden within the clue.
'refil' can be found hidden inside 'secure files'.
'refil' backwards is 'LIFER'.
(Other definitions for lifer that I've seen before include "Convicted murderer, say" , "Con" , "One with maximum sentence" , "person given sizeable bird" , "convict given longest sentence" .)